Liquid crystal materials have been used in the past for temperature sensing purposes. An example of one liquid crystal temperature sensor is disclosed in U.S. Pat. No. 4,140,016. In that patent the temperature sensor employs nematic liquid crystal with an optically active ingredient to cause the nematic liquid crystal to twist so that light ordinarily would be reflected by the liquid crystal material when the temperature thereof is below a so-called transition point temperature or clearing point temperature (the same being used interchangeably herein), whereby the liquid crystal material is in a mesomorphic phase. However, on reaching and exceeding the transition or clearing point temperature, the liquid crystal enters an isotropic phase and becomes substantially optically transparent. The transition temperature can be selected as a function of the ingredients and proportions thereof which are employed to formulate the liquid crystal material. One disadvantage encountered with prior liquid crystal temperature sensors has been the difficulty in reading the same due to inadequate lighting.
Other prior liquid crystal temperature sensor devices have used cholesteric liquid crystal material which ordinarily undergoes a change in apparent color as a function of temperature. One disadvantage frequently encountered with cholesteric temperature sensing devices is the relatively slow reversing, indeed sometimes the non-reversible nature, of the material, for cholesteric liquid crystal frequently is known to have a relatively long relaxation and/or memory characteristic. Another disadvantage with cholesteric liquid crystal temperature sensors is the relatively low temperature maximum of, say, on the order of 100.degree. C. to which the same would be operatively responsive.
The encapsulation or otherwise containment of liquid crystal material, particularly operationally nematic liquid crystal material, in volumes formed in a containment, encapsulating or support medium, is disclosed in the above referenced patent and applications. Moreover, the liquid crystal material and containment medium therein disclosed cooperate such that in the absence of a prescribed input the containment medium tends to distort the natural liquid crystal structure to a so-called curvilinear or distorted alignment. The extraordinary index of refraction of the liquid crystal material, which occurs in the absence of such input, is greater than the index of refraction of the containment medium; and, therefore, incident light impinging on the contained liquid crystal material will tend to be scattered preferably substantially isotropically. However, in the presence of a prescribed input, such as an electric field, the liquid crystal structure tends to align with respect to such input; the ordinary index of refraction of the liquid crystal material preferably is fairly closely matched to that of the containment medium; and, accordingly, the account of scattering (or absorption) of the incident light is reduced. Also disclosed in application Ser. No. 477,138, is a reflector arrangement which reflects a fairly large amount of the isotropically scattered light back to the liquid crystal material for further isotropic scattering thereby. Such reflection increases or enhances the effective brightness of the scattering contained liquid crystal material. The principle of total internal reflection can be relied on to achieve such reflection characteristic and, thus, the desired enhanced brightening.
A possible disadvantage of various media that have been used to encapsulate or to contain liquid crystal material is that such media may encounter substantial and/or relatively rapid discoloring, e.g. browning, when subjected to relatively high temperatures. Another possible disadvantage is that at relatively high temperatures the liquid crystal may dissolve into the medium.
Representative objects of the invention are to provide a liquid crystal temperature sensor that has a wide range of temperature responses, is capable of having high temperature response in particular, rapidly responds to temperature, has a sharp transition between mesomorphic and isotropic phases, is relatively promptly reversible, has the ability to be formulated to provide relatively widely different transition temperatures, and is relatively long lasting; and as to the containment medium, in particular, which contains plural volumes of liquid crystal, does not encounter discoloring or at least rapid discoloring of the containment medium and does not find the liquid crystal dissolving therein. Other objects are to provide improved liquid crystal materials, containment media, methods and techniques for sensing and indicating temperature as well as methods and techniques for formulating such liquid crystal material and devices employing the same.